VICTOR G. BRUCE 135 



cold, suggesting that the step in the cycle which is blocked occurs soon after 

 division. 



The results obtained with S. typhimurium and Amoeba suggest that the 

 blocking effects of temperature changes is less complete and that repeated 

 temperature changes are necessary for synchronization. In the case of 

 *S. ti/phiinurium the step which is affected probably occurs fairly early in 

 the division cycle. In the repeated temperature shift exjjeriment the bursts 

 of cell divisions occur within an 8- to 10-minute interval and this is ex- 

 plained as arising from individual differences between the cells. If the 

 temperature cycling causes an accumulation of cells in a phase immedi- 

 ately following division, and if, when the cells get out of this phase, they 

 take varying lengths of time to complete division, there will be some spread 

 in the time interval during which the cells divide. 



Probably there is more than one point in the division cycle at which the 

 cells can be blocked, even by temperature changes alone, and it would 

 be pure speculation to discuss the blocking mechanism in any more detail. 

 It appears however that the mechanism whereby synchronization is ef- 

 fected is based on ciualitative effects of the temperature changes on the 

 metabolism of the cell, such as might be expected to occur if there were 

 large differences in the temperature coefficients of different phases of the 

 cell cycle. 



SUMMARY 



Recent work on the synchronization of the division cycle of bacteria 

 and protozoa is reviewed and those systems in which synchronization has 

 been effected by means of temperature changes are discussed in some de- 

 tail. Recent ideas of the division cycle of the cell are discussed and, in 

 particular, attention is drawn to the probable existence of one or more 

 critical steps in the cycle which are particularly sensitive to being blocked. 

 The effects of temperature changes in inducing synchronization are con- 

 sidered as probably arising from the prevention of the completion of a 

 specific step in the cycle without affecting cells which have completed this 

 step. 



REFERENCES 



1. Earner, H. D. .\nd S. S. Cohen. Synchronization of a thymineless mutant of 

 Escherichia coli. J. Bact. 72: 115, 1956. 



2. Bruce, V. G., K. G. Lark and 0. Maal0e. Turbidimetric measurement on syn- 

 chronized Salmonella typhimurium cultures. Nature 176: 563, 1955. 



3. Bruce, V. G. and O. Maal0e. The ultraviolet inactivation of synchronously divid- 

 ing Salmonella typhimurium. Post irradiation effects on viability. Biochim. et 

 Biophys. acta 21: 227, 1956. 



4. Falcone, G. and W. Szyb.\lski. Biochemical studies on the induction of syn- 

 chronized cell division. Exper. Cell. Res. 11: 486, 1956. 



